The present invention relates to motor speed controls suitable for use in controlling and varying the speed of a motor driven kitchen appliance. Although speed control circuits for household kitchen appliances are well known in the art, several of the prior art speed control circuits have suffered from certain disadvantages.
U.S. patent Aaplication No. 916,189 to Ponczek et al., filed June 16, 1978, now abandoned, and assigned to the assignee of the instant application discloses a speed control circuit for use in an electric kitchen appliance, such as a mixer, which employs a mechanical governor as the sensing element and controls a triac, which in turn controls current flow to an appliance motor. It is clear that such a hybrid mechanical-electrical arrangement, while an improvement over previous systems, still leaves much to be desired, in that mechanical components which are subject to wear are employed in the system.
Another speed control system, also assigned to the assignee of the instant application, is disclosed in U.S. Pat. No. 4,227,128 to Cockroft et al. Cockroft et al. is directed to another speed control arrangement wherein the motor speed at which a mechanical governor switches is simultaneously altered as the average conduction angle of a triac is changed by movement of a single control knob. This device also suffers from some of the same disadvantages as the first mentioned mechanical governor/triac hybrid controller.
A relatively complex speed control circuit employing a multiple toothed pulse or interrupter wheel in association with a magnetic pickup as a speed sensing element, which is assigned to the assignee of the present application, is disclosed in U.S. Pat. No. 4,326,153 to Contri. It should be noted, however, that the Contri circuit requires the use of a 24-tooth interrupter wheel which adds additional expense to the appliance.
Accordingly, there is a need for a relatively inexpensive speed control circuit which does not require modification of the mechanical drive components of a home appliance. The electronic control unit should be adapted to be assembled and tested separately from other appliance components, such as the drive motor before the final assembly.